The present invention relates to a method and an arrangement for correcting, during pulsed gas metal-arc welding with a continuously fed electrode and material transfer from the electrode to the workpiece essentially in the form of non-short-circuiting drops, the power supply for the welding process on the basis of the occurrence of brief short circuits between the electrode and the workpiece. The correction is carried out in a regulating process in which the power supply is in the main regulated depending on the arc voltages so that a constant arc length is obtained. The correction facilitates welding using a very short arc.
In gas metal-arc welding using a continuously fed fusible electrode, usually called MIG/MAG welding, the workpiece is heated principally by the arc. The electrode is heated on the one hand by the power generated when the welding current flows through the electrode projection, that is to say the free electrode end between the contact nozzle, where the current transfer to the electrode takes place, and the arc, and on the other hand by the arc itself. Basic regulation of the welding process is aimed at obtaining a fusion rate of the electrode that corresponds to the electrode feed rate. Further aims of regulation may be, for example, to influence the quantity of heat supplied to the workpiece.
MIG/MAG welding is carried out in one of three states. In the case of short-arc welding, the material transfer from the electrode to the workpiece takes place by means of large short-circuiting drops, as shown in principle in FIG. 2. By virtue of the fact that the process consists of alternating arcs and short-circuiting drop transfers, the average voltage between the electrode and the workpiece is low and the heat transfer to the basic material is thus moderate. When the power supplied is increased, the mixed-arc range is entered, in which the material transfer is carried out by means of a mixture of short-circuiting and non-short-circuiting drops. The result is an unstable arc with a great deal of welding splash and welding smoke. Welding in this range is usually avoided. When the power supply is sufficiently great, the process enters the spray range, in which the material transfer is carried out by small finely-distributed drops without short-circuits, as shown in FIG. 3. The quantity of splash is clearly lower than in the case of short-arc welding. The heat supply to the basic material is greater in this case and the method is suitable principally for thicker material to be welded.
The third state is called pulsed welding and involves using advanced regulation to control the separation itself of the drops with the aid of a suitable current pulse. Each pulse separates a drop and the drops are sufficiently small so as not to short-circuit. This method, which is often called synergic pulsing, affords advantages from the spraying range in the form of low welding splash without the disadvantages of the great heat transfer.
So as to obtain stable conditions with uniform welding results even in the case of varying gun-guidance, the arc length is regulated. A method of doing this is described in EP 387 223. In this known method, the arc voltage is measured and compared with a reference value, and the current for the welding process is regulated on the basis of the calculated voltage difference so that the fusion rate increases when the arc voltage is lower than the reference value and decreases in the opposite case.
In the welding of thin plate, a low heat supply to the basic material and thus a short arc length are required in order to reduce the risk of burning through. At the same time, freedom from splashing is as always highly desirable. Pulsed welding gives freedom from splashing but greater heat than short-arc welding. It is therefore desirable to weld using the shortest possible arc length without the material transfer taking place via short-circuiting drops which explode easily and thus cause splashing.
Another reason for keeping the arc short is that a long arc destroys the gas protection in the case of, for example, aluminium welding.
EP 0 170 248 describes a welding machine for pulsed MIG/MAG welding, in which the equipment is regulated depending on the difference between a preset desired value for the number of drop short-circuits per unit of time and the actual number of short-circuits taking place. This regulating method gives a short arc and thus low heat transfer to the workpiece. One disadvantage, however, is that it involves completed drop short-circuits with the attendant risk of welding splash. A further disadvantage is that, with this method, no arc-length regulation is possible if it is desired to weld using a longer arc and thus a greater heat supply to the workpiece takes place.
A further problem with pulsed welding is the requirement for very accurate parameter settings. This may cause great problems when access is poor and it is necessary to turn the welding gun during welding in confined spaces. It is then very difficult to maintain a constant height above the workpiece. In this case, it is important to use a regulating method that can handle great deviations.
The object of the present invention is thus to make available a method and an arrangement for pulsed welding that completely or partly eliminate the problems of the known art.
The invention comprises a method and an arrangement for pulsed gas metal-arc welding with an electrode fed to the welding site continuously at a given rate, the fusion conditions being adapted in such a manner that the material transfer from the electrode to the workpiece takes place essentially without short-circuiting drops by the welding current being periodically increased to a pulse current of such size and duration that the current density in the electrode causes sufficient electromagnetic forces in order to separate one drop for each pulse, and where the arc length is stabilized by regulating the power supply for the arc by means of a regulator mainly on the basis of a measured arc voltage, characterized in that short-circuiting of the arc is detected as arc-voltage decreases and/or current increases, and in that the fusion power supplied is increased by the arc-length regulator being given an additional increase for each short-circuit detected.
As a result of the arc pressure, the molten pool is always in motion. When the arc length is shortened, the electrode end will briefly come into contact with the moving molten pool. These brief contacts do not normally lead to any material transfer, however, as the drop is retained on the electrode by forces directed towards the electrode. These repellant forces may, for example, be connected with the vaporization of principally oxidized alloying materials from the surface of the fusible electrode end. By detecting these brief short-circuits and, on the basis of their occurrence, increasing the power for the arc, the fusion rate and thus the arc length are increased. In this way, the procedure and the arrangement according to the invention make it possible to weld with minimum arc length without ending up in the short-arc range.
The method facilitates considerably synergic pulsing with a very short arc and thus makes possible virtually splash-free welding, even of thin plate. The method also offers considerably reduced sensitivity for parameter-setting accuracy and thus makes possible welding in situations that are very difficult to deal with.
Further advantageous features of the invention are indicated in the subclaims.